I think that you mean well, but I have to say, this is a lot more than DH not being able to enjoy the event. There has to be a reality check as to what an ordeal this will be for a 93 year old, frail woman. There is a 7 hour drive there, along with who knows how many stops necessary, going to the rehearsal dinner, getting around at the wedding ceremony, getting around at the reception, staying in a strange hotel, and getting around there, then a 7 hour drive back. I'm sure there are people in their nineties who could handle this, but this is a 93 year old who is mobility limited and confused. When elderly people are confused, putting them into strange situations makes them more confused.
I spent a year taking care of my 93 year old mother before she died, and 4 years taking care of my 96 year old father. I cannot imagine a woman who has the problems that this woman has been described as having, without it having major effects on her. My father is worn out after being taken to the eye doctor, and he is in a wheelchair the entire time. What happens if they get there and the woman is too tired to enjoy any of it or just doesn't feel well? This just seems like a really bad idea.
You're right the most important thing isn't whether DH enjoys the event, but there needs to be a realistic view of what it will be like for the grandmother, and I don't feel that asking her if she can handle it is a guarantee of anything. She won't know whether she can handle it until she is 7 hours from home.